Rat antimouse monoclonal antibodies (MAb) which block or augment killing of rat tumor target cells by murine cytotoxic T lymphocytes (CTL) will be obtained. Whether MAb blocks at the stage of crawling activity, specific antigen recognition or adhesion or delivery of the lethal hit will be determined. Analogous mixed-lymphocyte reaction (MLR) blocking MAb will also be obtained. Antigen-negative mutant CTL clones will be selected by complement-mediated lysis or in the fluorescence-activated cell sorter (FACS). Antigens which are sites for blocking and mutations and which lead to loss of CTL activity are implicated in lytic mechanisms. The molecular weight and subunit structure of these molecules will be studied after radiolabeling and immunoprecipitation. Clonal variation suggestive of antigen receptors will be examined by peptide mapping. Alpha-chains of Mac-1 and LFA-1 antigens will be compared by carbohydrate and tryptic peptide map differences, and for maturation or differentiation-related changes in expression in lymphocytes, granulocytes and monocytes. This basic research on lymphocyte antigen structure and function should suggest many avenues for therapy and diagnosis in neoplasms and immune disorders. In the past year, peptide mapping has shown that the alpha subunits of Mac-1 and LFA-1 are quite different, whereas the beta subunits appear identical. The alpha and beta subunits have been dissociated, and it was found with monoclonal and conventional sera that the alpha-chains are noncross-reactive and the beta-chains are completely cross-reactive. Monoclonal antibodies binding to the alpha-chain of LFA-1 inhibit killing whereas those binding to the beta-chain do not, suggesting a functional site on the alpha-chain. An antigen-specific CTL line has been found to be LFA-1+, Lyt 2- and strongly inhibitable by anti-LFA-1 but not by Lyt 2 MAb, showing that Lyt 2 is not essential for killing.